1. Field of the Invention
The present invention relates to a turbine casing and a valve casing, and particularly to a turbine casing or valve casing to be used for a steam turbine.
2. Description of the Related Art
Turbine casings and valve casings used in steam turbines for thermal power generation are complicated in their shape and, therefore, they are fabricated by use of cast steels in many cases. These high-temperature members are required to be high in high-temperature strength and creep rupture strength. Further, where a cast steel is used, these members are required to have excellent weldability, because of the need to repair their defective portions by welding.
In order to meet these requirements, there have come to be used casing materials which are high in strength and excellent in weldability, such as the casing material disclosed in JP-A-1996-209293.
Other than the just-mentioned material, there are known the 2.25Cr-2W—Mo—V cast steels to be used as an alternative to high-Cr heat resisting steels, such as those described in JP-A-2001-115230 and JP-A-2001-059130, and the Cr—Mo—V—W cast steels enhanced in weldability through improvement of impact resistance property, such as those described in JP-A-2000-273570 and JP-A-1998-259449.